The present invention relates to an apparatus and method for selectively increasing the photosensitivity of selective portions of optical fibers. Specifically, the present invention comprises an apparatus for rapidly diffusing hydrogen or deuterium into selective regions of silica glasses to increase the photosensitivity of these glassy materials, and in particular of optical fibers. In a particular embodiment, the apparatus of the present invention is used in an in-line system for the manufacture of Bragg gratings.
Optical fibers and optical fiber devices are widely used in signal transmission and handling applications. Optical fiber-based devices are vital components in today""s expanding high-volume optical communications infrastructure. Many of these devices rely on fiber Bragg gratings (FBG""s) to perform light manipulation. An FBG is an optical fiber with periodic, aperiodic or pseudo-periodic variations of the refractive index along its length in the light-guiding region of the waveguide. The ability to produce these refractive index perturbations in a fiber is necessary to manufacture FBG""s and, hence, a number of optical components, such as optical sensors, wavelength-selective filters, and dispersion compensators.
Gratings are written in optical fiber usually via the phenomenon of photosensitivity. Photosensitivity is defined as the effect whereby the refractive index of the glass is changed by actinic radiation-induced alterations of the glass structure. The term xe2x80x9cactinic radiationxe2x80x9d includes visible light, UV, IR radiation and other forms of radiation that induce refractive index changes in the glass. A given glass is considered to be more photosensitive than another when a larger refractive index change is induced in it with the same delivered radiation dose.
The level of photosensitivity of a glass determines how large an index change can be induced in it and therefore places limits on grating devices that can be fabricated practically. Photosensitivity also affects the speed that a desired refractive index change can be induced in the glass with a given radiation intensity. By increasing the photosensitivity of a glass, one can induce larger index perturbations in it at a faster rate.
The intrinsic photosensitivity of silica-based glasses, the main component of high-quality optical fibers, is not very high. Typically index changes of only xcx9c10xe2x88x925 are possible using standard germanium doped fiber.
However, it has been observed that by loading the glass with molecular hydrogen before irradiating it with actinic radiation, one can increase significantly the photosensitivity of the glass. Exposing Ge-doped silica optical fibers to hydrogen or deuterium atmospheres at certain temperatures and pressures photosensitizes the fibers. Index changes as large as 10xe2x88x922 have been demonstrated in hydrogenated silica optical fibers.
Prior references have emphasized upper limits on the temperature for such hydrogen loading. For example, U.S. Pat. Nos. 5,235,659 and 5,287,427 discuss a method for exposing at least a portion of a waveguide at a temperature of at most 250xc2x0 C. to H2 (partial pressure greater than 1 atmosphere (14.7 psi), such that irradiation can result in a normalized index change of at least 10xe2x88x925. U.S. Pat. No. 5,500,031, a continuation-in-part of the above-mentioned ""659 patent, speaks of a method of exposing the glass to hydrogen or deuterium at a pressure in the range of 14-11,000 psi and at a temperature in the range 21-150xc2x0 C. The parameters described in these references are probably typical for hydrogen-loading an optical fiber
The ""031, ""659 and ""427 references point out problems with hydrogen loading methods in which temperatures exceed 250xc2x0 C., or even 150xc2x0 C. In teaching away from higher temperatures, the ""659 Patent indicates that at high-temperatures xe2x80x9ctypical polymer fiber coatings would be destroyed or severely damagedxe2x80x9d (column 1, lines 51-54). It further emphasizes the fact that xe2x80x9cthe prior art high temperature sensitization treatment frequently increases the optical loss in the fiber and/or may weaken the fiberxe2x80x9d (column 1, lines 54-56). Finally, the ""659 patent differentiates itself from the prior art by stating that a high temperature treatment involves xe2x80x9ca different physical mechanismxe2x80x9d than does a low-temperature treatment. For example, U.S. Pat. No. 5,235,659 explicitly indicates that temperatures of xe2x80x9cat most 250xc2x0 C.xe2x80x9d should be used.
It has been observed that at higher temperatures the polymer coating, (usually an acrylate material), that protects the glass from harmful chemical reactions in a normal environment will degrade or oxidize (bum). Coatings that have degraded or oxidized and lost their protective value need to be removed and replaced, which can be a difficult and expensive process. Uncoated fiber is fragile, and requires great care during handling.
Most of the gratings written today by industry involve about 5 cm (2 inches or less) of the length of a fiber, depending on the type of grating to be written. Traditionally, it has been taught to place an entire length of optical fiber in a vessel containing hydrogen or deuterium atmospheres at certain temperatures and pressures. The grating manufacturing process usually entails a first process of placing a fiber spool in a hydrogen or deuterium containing vessel, placing the vessel in an oven and loading the entire fiber through the polymer coating.
To achieve the desired level of hydrogen in fiber with conventional hydrogenating methods (xcx9c1 ppm), one will typically expose fiber to a hydrogen atmosphere for several days and, in some cases, for several weeks. Exemplary exposures such as 600 hours (25 days), 21xc2x0 C., at 738 atm or 13 days, 21xc2x0 C. at 208 atm are reported as typical. Obviously, such long exposures extend the time required to fabricate optical devices that rely on photosensitive glass. Because of the long duration needed for traditional fiber hydrogenation, several pressure vessels are needed in a high-volume production environment to increase throughput and avoid idle time. These vessels are costly to install safely and increase the potential for serious accidents, especially when multiple vessels with separate control valves and gas supply cylinders are involved. Although installing multiple vessels can increase production throughput, the hydrogenation process hampers grating fabrication cycle time, thus new product and specialty product development time can be compromised severely.
Once the length of fiber has been hydrogen-loaded, the coating is stripped (mechanically, chemically or by other means) from the area where the grating is to be written. A technician then uses a source of actinic radiation to write each grating individually. The fibers are then annealed by again heating the fiber to reduce the degradation curve of the gratings. The portion of the fiber that was stripped is then recoated.
The traditional Bragg grating manufacturing processes are slow and do not lend themselves to mass manufacturing. The traditional hydrogen loading techniques require that the entire length of fiber be subject to the hydrogen loading and heating cycles. The need to expose the entire fiber may result in optical effects on the fiber and places constraints on materials, such as fiber coatings, that may be used. One negative effect of hydrogen loading at higher temperatures is that it may increase the optical loss characteristics of an optical fiber. Furthermore, high-temperature heating cycles may deteriorate optical fiber coatings.
The need remains for a process and enabling machinery that is amenable to higher speed mass manufacturing and that reduces deleterious effects on the optical medium.
The present invention is directed to an apparatus and a method for selectively exposing only a selected portion of an optical fiber to a hydrogen atmosphere loading process. The apparatus includes a loading chamber that encloses at least the selected portion of the optical fiber and contains a hydrogen gaseous atmosphere. The chamber includes a heating element that locally heats the hydrogen atmosphere surrounding the selected portion. In high-temperature embodiments, the heating element heats the hydrogen atmosphere to a temperature of at least 250xc2x0 C. Also, the loading chamber may be a pressure chamber capable of containing a pressurized atmosphere. In one particular embodiment, the chamber is designed to contain pressures up to 3,000 psi.
The selected portion may be a midspan portion of a continuous length of fiber, where the loading chamber encloses only the selected portion of the continuous optical fiber.
In one exemplary embodiment, the loading chamber comprises a tube concentrically surrounding only the selected portion of the optical fiber. Gas seals positioned at ends of the tube contain the hydrogen atmosphere while allowing passage of the length of optical fiber. Gas seals also may be attached to end sections of the selected portion of the optical fiber, so the tube becomes sealed as the fiber is placed into position. In another embodiment, the loading chamber comprises a vessel enclosing the entire optical fiber. The vessel may further include a reel-to-reel arrangement, wherein end portions of the length of the optical fiber are wound on laterally spaced reels and the selected portion is suspended midspan. Where the optical fiber is held in a reel-to-reel arrangement, the heating region may be positioned at the midspan portion of the optical fiber.
In yet another embodiment, the apparatus includes a first and a second clamping vessel blocks. The vessel blocks have pockets that define the loading chamber when the vessel blocks are clamped together. The fiber is positioned between the blocks and the blocks close about the selected portion of the fiber to be loaded. Elastomeric re-closable seal may be used to clamp the ends of the selected portion and to contain the gas atmosphere. Alternatively, at least one pressure seal adapted to help contain a hydrogen atmosphere within the loading chamber may be physically affixed to the optical fiber.
The elastomer may be a curable elastomer. The pressure seal may be located at a boundary between the selected portion of the optical fiber and a non-selected portion and/or at the ends of a cooling area.
Gas inlet and vent lines may inject and vent the gaseous atmosphere in the loading chamber. A pre-heating chamber may be used to heat the hydrogen atmosphere prior to introducing the hydrogen atmosphere into the loading chamber.
Particular embodiments include cooling regions that cool or dissipate heat along the portions of the fiber adjacent to the selected portion. A cooling device may regulate the temperature of the cooling region. In one embodiment, cooling tubes are attached to ends of a loading chamber tube. The cooling tubes may include seals that separate the cooling areas from the loading chamber. An embodiment further includes a mechanism that allows the fiber to be moved from the loading chamber to the cooling region. In one specific embodiment, the mechanism comprises a movable magnet and a magnetic body attached to the fiber.
A method in accordance with the present invention for increasing the photosensitivity of a selected portion of an optical fiber includes the step of placing at least the selected portion of the optical fiber in a hydrogen-containing atmosphere. The term hydrogen atmosphere in the present description is intended to include atmospheres including H2, D2, tritium, or molecules such as HD that combine these isotopes of hydrogen. The volume of the hydrogen-containing atmosphere immediately surrounding only the selected portion of the optical fiber is heated to a temperature of at least 250xc2x0 C. The selected portion of the optical fiber is exposed to the heated volume of the hydrogen-containing atmosphere at a temperature of at least 250xc2x0 C. for a predetermined time.
In a particular embodiment of the process, only the selected portion of the optical fiber is placed in the hydrogen-containing atmosphere. Pressure seals may be located at a boundary between the selected portion of the optical fiber and a non-selected portion. The pressure seals even may be physically affixed to the optical fiber to help contain a gaseous atmosphere within the loading chamber. In one particular embodiment, the seals are re-closable seals including an elastomeric collet.
The temperature of adjacent portions of the optical fiber may be controlled, either by heat dissipation or by active cooling.
After the step of exposing, the method may further include the step of rapidly changing the atmosphere surrounding the selected portion after the exposing step. This may be done by venting the hydrogen-containing atmosphere from the loading chamber or by physically removing the selected portion from the loading chamber.
The method may further include the step of rapidly cooling the selected portion of the optical fiber after the predetermined time. This may be done, for example, by replacing the hydrogen atmosphere with a cooled inert gas or by physically removing the selected portion from the loading chamber into a cooling chamber.